1. Field of the Invention
The present invention relates to a shaft for a golf club.
2. Description of the Related Art
So-called steel and carbon shafts have been known as a shaft for a golf club. A material of the carbon shaft is a CFRP (a carbon fiber reinforced plastic).
Many carbon shafts are manufactured by a so-called sheet winding process. In the sheet winding process, a prepreg sheet having a fiber and a matrix resin is used. In the process, a sheet constituted by the prepreg is wound around a metallic core body and the matrix resin is then cured by heating, and the core body is pulled out after the curing. By the process, a shaft is formed by winding and curing the prepreg sheet.
The carbon shaft usually has a straight layer and a bias layer (an angle layer). The bias layer is mainly related to a twist rigidity of the shaft. A shaft torque value (which is also referred to as a torque value) has been known as an index representing the twist rigidity of the shaft. A twist angle formed by an application of a torque on a constant condition is the torque value. The lower torque value means the higher twist rigidity of the shaft.
A high twist rigidity can suppress the twist of the shaft which is caused by a shock power of an impact. The high twist rigidity can improve a orientation of a ball.
An increase in an amount of a fiber of the bias layer can contribute to enhance the twist rigidity. On the other hand, a weight of the shaft is increased. The increase in the weight of the shaft causes a decrease in a head speed and a reduction in a flight distance. By an increase in a coefficient of elasticity of a fiber in the bias layer, similarly, it is possible to enhance the twist rigidity. In this case, however, a strength of the shaft tends to be reduced.
Japanese Laid-Open Patent Publication No. 9-234256 has disclosed a shaft in which a partial bias layer is provided in tip and butt end portions in addition to a bias layer provided over a full length of the shaft. Japanese Laid-Open Patent Publication No. 2002-126141 has disclosed a shaft in which a stuck sheet laminated and integrated by sticking two bias sheets is laminated on an outer layer.